Maintaining communication sessions

ABSTRACT

Communication occurs between a server and a terminal. The terminal establishes a first communication path with the server via a first allocated address. The server allocates an identification reference to a communication session upon initiation of said communication session. During the communication session the first communication path is closed and replaced by a second communication path with said server via a second allocated address. The server maintains said communication session after replacement of the first communication path with the second communication path by identifying said identification reference and communicating with said remote station via said second allocated address.

FIELD OF THE INVENTION

The present invention relates to communicating between stations.

DESCRIPTION OF THE RELATED ART

Protocols are known for use in communications between stations over anetwork such as the Internet. Communication over the Internet isfacilitated by allocating an Internet Protocol (IP) address to everystation. An IP header is then wrapped around every data packet and isused to route the packet to its destination.

When a mobile device accesses the Internet via a mobile telephony orother radio network, it is allocated an IP address by the network whichis valid for the cell that it is in. However this poses a problem whenthe mobile device moves between cells or networks, since the device canno longer receive data arriving to its allocated IP address.Alternatively, a terminal accessing the Internet via a wired connectionis often allocated an IP address by its Internet Service Provider (ISP).If the connection to the Internet is broken then the ISP may allocate anew IP address to the terminal. Again, this will cause a break incommunications.

BRIEF SUMMARY OF THE INVENTION

According to an aspect of the present invention there is provided amethod of communicating between a server and a terminal, wherein saidterminal establishes a first communication path with said server via afirst allocated address; the server allocates an identificationreference to a communication session upon initiation of saidcommunication session; during said communication session said firstcommunication path is closed and replaced by a second communication pathwith said server via a second allocated address; and the servermaintains said communication session after replacement of the firstcommunication path with the second communication path by identifyingsaid identification reference and communicating with said remote stationvia said second allocated address.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 illustrates a networked environment;

FIG. 2 illustrates a prior art method of supplying data from a server toa terminal over a telephony network;

FIG. 3 shows a prior art graph of data against time;

FIG. 4 illustrates a typical performance of TCP over a mobile telephonynetwork;

FIG. 5 shows a real time data provider shown in FIG. 1;

FIG. 6 details a real time data server shown in FIG. 5;

FIG. 7 details steps carried out by the real time data server shown inFIG. 6;

FIG. 8 details the contents of the memory shown in FIG. 6;

FIG. 9 details a session item shown in FIG. 8;

FIG. 10 details steps carried out during FIG. 7 to execute real timedata server instructions;

FIG. 11 illustrates the structure of a typical datagram;

FIG. 12 details an MTP header shown in FIG. 11;

FIG. 13 details steps carried out during FIG. 10 to transmit datagrams;

FIG. 14 illustrates the process of transmitting data;

FIG. 15 details steps carried out during FIG. 13 to prepare atransactional datagram;

FIG. 16 details steps carried out during FIG. 13 to prepare a streameddatagram;

FIG. 17 details steps carried out during FIG. 10 to perform outputbuffer processing;

FIG. 18 details steps carried out during FIG. 17 to set an MTP header;

FIG. 19 details steps carried out during FIG. 10 to receive datagrams;

FIG. 20 illustrates the use of an MTP header field to measure connectionlatency;

FIG. 21 details steps carried out during FIG. 19 to processacknowledgements and state changes;

FIG. 22 details steps carried out during FIG. 21 to process an extendedacknowledgement;

FIG. 23 illustrates the reception of a datagram;

FIG. 24 details steps carried out during FIG. 19 to extract the datacontained in a received datagram;

FIG. 25 details steps carried out during FIG. 10 to process datagramsplaced in the transactional segment buffer;

FIG. 26 details steps carried out during FIG. 10 to process incomingstreamed datagrams;

FIG. 27 details steps carried out during FIG. 10 to perform backgroundprocessing;

FIG. 28 illustrates an extended acknowledgement;

FIG. 29 details steps carried out during FIG. 27 to update the datagramtransmission rate;

FIG. 30 details steps carried out during FIG. 10 to perform sessionmaintenance;

FIG. 31 details an application server shown in FIG. 5;

FIG. 32 details steps carried out by the application server shown inFIG. 31;

FIG. 33 details the contents of the memory shown in FIG. 32.

FIG. 34 details instructions executed by a process shown in FIG. 32;

FIG. 35 details steps carried out during FIG. 34 to send a selectiveupdate;

FIG. 36 illustrates examples of providing varying levels of servicedependent upon network conditions;

FIG. 37 details a PDA shown in FIG. 5;

FIG. 38 shows steps carried out by the PDA shown in FIG. 37;

FIG. 39 details the contents of memory shown in FIG. 38;

FIG. 40 details steps carried out during FIG. 38 to execute real timeapplication instructions;

FIG. 41 illustrates the calculation of resend latency; and

FIG. 42 details steps carried out during FIG. 40 to negotiate aheartbeat rate

WRITTEN DESCRIPTION OF THE BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1

FIG. 1 illustrates a networked environment in which the invention may beused. A Real Time Data Provider 101 provides data to a number ofterminals 102, 103, 104, 105, 106, 107, 108 and 109 via the Internet110. The data can be separated into at least two types. The first typeis streamed data, which comprises updates of certain information that auser of a terminal has indicated that he is interested in. This couldbe, for example, financial data such as stock prices or exchange rates,sports data such as the latest football scores, news items and so on. Asecond type of data is transactional data. This comprises any dataforming a transaction, which could be a financial transaction such asplacing a bid to trade stocks or placing a bet on a sports fixture.Transactional data can also include logging-on or user profileactivities.

The data is provided over a variety of networks, including radionetworks such as mobile telephony networks or wireless networks. A ThirdGeneration (3G) mobile telephony network, connected to the Internet 110,includes a gateway 111 which provides connectivity to a network of basestations. Terminals 102 and 103 are each connected to one of these basestations. A General Packet Radio Service (GPRS) gateway 112 is connectedto the Internet 110 and provides connection to a network of GPRS basestations. Terminals 104 to 106 are each connected to one of thesestations. A GMS gateway 113 is connected to the Internet 110, providingconnectivity for terminal 107. A terminal could, when possible, switchbetween connections as shown by dotted line 114.

Internet Service Provider (ISP) 115 is connected to the Internet 110 andprovides internet access for server 116, server 117 and a WirelessNetwork or Wireless Fidelity (WiFi) gateway 118. Terminal 108 has a linkto gateway 118. ISP 119 is connected to the Internet 110 and providesinternet access for computer systems 120, 121, 122 and 123 via wirelinks. Terminal 109 is connected by an ethernet wire link, possiblyusing a docking cradle, to computer system 122. Alternatively, server124 is connected directly to the Internet 110.

Thus there is a number of ways in which a terminal may link to theInternet 110 in order to receive data from RTDP 101. There are, ofcourse, other methods of connection and the rate of technologicaladvance means that in the future there will be further methods. Thisdescription should not be construed as limiting connection methods tothose described here. However, the number of methods makes the task ofproviding real time data difficult. While it is, for example, relativelyeasy to provide data quickly to terminals 108 and 109, terminals 102 to107 use relatively low bandwidth, high latency and high variabilityconnections over which it is very difficult to provide real time data.

Mobile telephony systems such as those provided by gateways 111 to 113are used to provide data. For example, mobile telephone users are ableto browse the Internet 110. However, the rate of data supply can beextremely slow. This is merely inconvenient when browsing. However, ifdata on the basis of which decisions are to be made is required, forexample financial data, it must be provided in a timely fashion. Thismeans that the data should arrive at the terminal quickly, andpreferably it should be possible to indicate to a user how up-to-datethe information is.

FIG. 2

FIG. 2 illustrates a prior art method of supplying data from a server toa terminal over a telephony network. A server 201 on an ethernet network202 supplies data packets to a first gateway 203, where the data packetsare placed on a high capacity data interconnect 204. A router 205receives these packets and supplies them to another network 206.Eventually the packets arrive at a telecoms gateway 207, where atelecoms provider can select which of several wireless networks tosupply the packets to. A GPRS gateway 208 then supplies the packets to aGPRS router 209, which routes the packets to the base station 210 towhich the terminal 211 is currently connected.

This journey across several networks is facilitated by the InternetProtocol (IP) which provides a header at the start of every packetdefining the destination IP address. Other information is also providedin the IP header, such as the size of the packet, but its primaryfunction is to define an address that gateways and routers can read, anddecide where the packet should be sent next. Packets are sentseparately, and may end up taking different routes. It is thereforepossible for packets to arrive out of order.

In order to maintain a dialogue between server 201 and terminal 211, anadditional protocol must be used. Most commonly, this protocol is theTransport Control Protocol (TCP). This enables a two-way link to be setup between two systems on the Internet 110. Messages are sent, and TCPprovides functionality such as acknowledging and resending data, ifnecessary, and reordering packets if they arrive in the wrong order. TCPwas designed to be used on networks that have a high data capacity andlow latency, but can suffer from congestion. However mobile telephonynetworks have different characteristics and TCP handles certain of thesecharacteristics in an ineffective way.

In the communication chain shown in FIG. 2, TCP (and other protocols)achieve effective communication across high-capacity parts of theInternet 110. However, the final link to terminal 211, over alow-capacity wireless connection, is extremely vulnerable. TCP fails toaddress these vulnerabilities effectively, since it was not designed forthat purpose.

FIG. 3

FIG. 3 shows a prior art graph of data against time for packets that aresent over the Internet 110. Graph 301 illustrates the headers of apacket sent using a transport protocol such as TCP. The Internet 110comprises many interconnected networks. As a packet is sent over eachindividual network, a local network protocol header 302 is attached toit, generally to transfer it from one part of the network to another. Atthe point of exit from the network, the network gateway will strip thelocal network protocol header 302, leaving the IP header 303. From thisthe next destination on a neighbouring network is determined (the routeruses various algorithms to work out the next intermediate destination).The local network protocol header is transient, and changes as thepacket traverses the Internet 110.

The IP header 303 defines the destination IP address for the packet.After this, there is the transport protocol header 304, which istypically used by the communication client and server to form aconnection over which communications can take place. Finally theremainder of the data packet 305 is the data payload. Some packets donot have data, and simply consist of signalling in the transport header304, for example an acknowledgement packet that tells the recipient thatsome data has been successfully received. Typically, though,acknowledgements are combined with data to reduce traffic.

An example of a transport protocol is TCP, as described with referenceto FIG. 2. TCP forms reliable connections and is often combined withhigher protocols such as the File Transfer Protocol (FTP) or HypertextTransport Protocol (HTTP).

FIG. 4

FIG. 4 (prior art) illustrates a typical performance of TCP over amobile telephony network. Graph 401 plots bandwidth 402 against time403. The upper line 404 shows theoretically available bandwidth over thenetwork, while the lower line 405 shows the use made of the bandwidthusing TCP.

TCP's performance is always less than 100%. When there are significantchanges in network availability, TCP compensates inefficiently, becauseits underlying mechanisms make assumptions about the network that areinvalid for a mobile connection. When bandwidth falls off, for exampleat point 406, the amount of data sent using TCP falls much faster,because data packets that have been lost need to be resent, resulting ina downward spiral of lost bandwidth. TCP cannot anticipate or compensatefast enough to avoid such inefficiencies.

When a disconnection occurs, such as at point 407, TCP takes a long timeto reestablish data flow when the link is reconnected. When using aterminal on a mobile telephony network, such disconnections arefrequent, for example when the user goes through a tunnel.

TCP presents another problem to real time data provision. When adisconnection takes place (as at point 407), a wireless service providerwill often perform a service known as “IP spoofing”. This involves aproxy server being used to maintain the TCP connection with a server,even though the wireless connection is broken. When the connection isreestablished data can be sent from where it is cached on the proxyserver to the terminal. The telecoms provider does this so that a datatransfer can continue, rather than being restarted every time theconnection is lost.

This operation is helpful for internet browsing and downloading of largefiles to mobile telephones. However, it presents two problems to RTDP101. The first is that if the telecoms provider caches a large amount ofstreamed data and sends it all to a terminal upon reconnection this canoverload the connection. This is especially inappropriate given thatmuch of it may be out of date. The second problem is that the RTDP 101might send transactional data to, for example, terminal 102 while it isdisconnected from 3G gateway 110. The 3G network, spoofing terminal 102,will acknowledge this data. However, if terminal 102 does not reconnect,which might happen for one of many reasons, then the cachedtransactional data will never be forwarded. This results in RTDP 101wrongly concluding that terminal 102 has received the data.

A further problem with TCP is that it is a connection-oriented protocol.When a client moves between wireless base stations its IP address canchange, resulting in a requirement to set up a new TCP connection. Thiscan interfere with communications. In particular, a secure transactioncould be terminated. This also prevents a terminal from using ahigher-bandwidth, lower latency network that may become availablewithout terminating a connection, for example when a terminal connectedto GPRS gateway 112 comes within range of 3G gateway 111, or moves intothe radius of a WiFi gateway 118.

FIG. 5

FIG. 5 shows RTDP 101 which comprises an application server 501 and areal time data server 502. The real time data server communicates with alarge number (potentially thousands) of terminals. It facilitatescommunications between the application server 501 and the terminals.Terminals can have a variety of types of connection, including highspeed WiFi or wire. The real time data server 502 manages communicationswith all these types of connections. A terminal need not be mobile totake advantage of the system.

The application server 501 receives data from a number of data feeds.These are illustrated by two-way arrows, as data is provided toapplication server 501 but the server may also send information back,for example details of a financial transaction or an informationrequest. Financial transaction services data feed 503 providescommunications for making stock-market-based transactions. Sportstransaction services data feed 504 provides communications for makingsports-based transactions. Financial data feed 505 provides real timeupdates of, for example, share prices and exchange rates, while sportsdata feed 506 provides real time updates of sports scores. News datafeed 507 provides news headlines and stories. It will be appreciatedthat the data feeds illustrated in FIG. 5 are representative of the typeof data that a Real Time Data Server might provide to clients. Otherdata types and feeds are contemplated and included in this description.

The application server 501 communicates with the real time data server502 over an outbound-initiated TCP-based link 508. The connectionbetween the two systems is made via a high-speed Gigabit Ethernetconnection. In other embodiments, the two servers could use the sameprocessing system. However, this provides less security.

The application server 501 is protected by a first firewall 509, so asto resist any security vulnerabilities that may exist in the real timedata server 502, which has its own firewall 510. The real time dataserver 502 takes data from the application server 501 and supplies it toterminals via the Internet 110 using a custom protocol called the MobileTransport Protocol (MTP). This protocol addresses the needs of real timedata services for mobile client terminals.

In the embodiment described herein the terminals are Personal DigitalAssistants (PDAs) such as PDA 511. These are small portable devicesincluding a display screen 512, control buttons 513, a speaker 514 and amicrophone 515. The display 512 may be touch-sensitive, allowing the PDA511 to be controlled using a stylus on the screen instead of buttons513. A typical PDA is supplied with software providing the functionalityof, inter alia, a mobile telephone, word processing and otheroffice-related capabilities, a calendar and address book, email andinternet access, games, and so on. The skilled reader will appreciatethat the PDAs illustrated in this document are not the only terminalsthat can be used. For example, a mobile telephone with enough storageand memory could be used, or other devices which can communicate overmobile telephony networks.

PDA 511 may communicate with the real time data server 502 to obtainaccess to data provided by any of data feeds 503 to 507, or to obtainsoftware downloads for installation. The application server 501facilitates several different types of service. In particular, theefficient provision of multiple types of data having differentcharacteristics is enabled using the custom protocol MTP.

The two main types of data are transactional data and streamed data. Fortransactional data, a two-way communication between the PDA 511 and thereal time data server 502 facilitates the making of a securetransaction. Data delivery must be guaranteed even if a connection isbroken. Such data may be several kilobytes for each message, requiringmultiple datagrams to be transmitted before a message is complete. Thesepackets, or datagrams, must be reassembled in the right order beforeuse.

Streamed data comprises updates, for example of financial or sportingdata. These may be provided at a fixed regular rate, or may be providedat an irregular rate as the data becomes available. Each update ormessage is contained in a single datagram (although a datagram maycontain more than one message). For this reason it is not necessary forstreamed datagrams to be ordered at the terminal.

Because of these different data types, each of which has its own issuesto be addressed, MTP provides two types of data communication,transactional communication and streamed communication. It facilitatescommunication of both types over the same communication link. The datatypes are differentiated, such that the bandwidth utilisation ismaximised without compromising transactional communications. Itspecifically addresses the need for bandwidth efficiency, latencymeasurement, multiple data types and continuous updates over a lowbandwidth, high latency, high variability wireless mobile link. Also,because by its nature a mobile terminal such as a PDA has low storageand memory capabilities, it minimises the computational requirements ofthe terminal.

FIG. 6

FIG. 6 details real time data server 502. It comprises a centralprocessing unit (CPU) 601 having a clock frequency of three gigahertz(GHz), a main memory 602 comprising two gigabytes (GB) of dynamic RAMand local storage 603 provided by a 60 Gb-disk array. A CD-ROM diskdrive 604 allows instructions to be loaded onto local storage 603 from aCD-ROM 605. A first Gigabit Ethernet card 606 facilitates intranetconnection to the application server 501. The intranet can also be usedfor installation of instructions. A second Gigabit Ethernet card 607provides a connection to Internet 110 using MTP.

FIG. 7

FIG. 7 details steps carried out by real time data server 502. At step701 the real time data server 502 is switched on and at step 702 aquestion is asked as to whether the necessary instructions are alreadyinstalled. If this question is answered in the negative then at step 703a further question is asked as to whether the instructions should beloaded from the intranet. If this question is answered in theaffirmative then at step 704 the instructions are downloaded from anetwork 705. If it is answered in the negative then at step 706 theinstructions are loaded from a CD-ROM 707.

Following either of steps 704 or 706 the instructions are installed atstep 708. At this point, or if the question asked at step 702 isanswered in the negative, the instructions are executed at step 709. Atstep 710 the real time data server is switched off. In practice thiswill happen very infrequently, for example for maintenance.

FIG. 8

FIG. 8 details the contents of memory 602 during the running of realtime data server 502. An operating system 801 provides operating systeminstructions for common system tasks and device abstraction. TheWindows™ XP™ operating system is used. Alternatively, a Macintosh™,Unix™ or Linux™ operating system provides similar functionality. Realtime data server instructions 802 include MTP instructions andinstructions for providing MTP status information to the applicationserver 501. Session data 803 comprises the details of every session,such as session item 804, currently maintained by the server 502. Eachclient terminal that is currently logged on has a session, and when asession starts an area of memory is allocated to it in which variables,specific to each user, are stored. Other data includes data used by theoperating system and real time data server instructions.

FIG. 9

FIG. 9 details an individual session item 804 shown in FIG. 8. Eachsession item includes a session ID 901 and session state variables 902,indicating whether the session is starting, ongoing, stalled,reconnecting or disconnecting. Each item also includes transmitter data903 and receiver data 904, since MTP provides two-way communication.Transmitter data 903

-   -   includes a transactional segment buffer 905, a streamed segment        buffer 906 and prioritised message queues 907. Receiver data 904        includes a transactional segment buffer 908 and prioritised        message queues 909.        FIG. 10

FIG. 10 illustrates step 709 at which the real time data serverinstructions are executed. This step comprises a number of separateprocesses that effectively occur in parallel. The concurrency of theseprocesses is achieved by a mixture of concurrent threads and sequentialprocessing, details of which will be known to those skilled in the art.In particular, although the processes may be described in terms ofcommunications with a single client, PDA 511, they should be understoodto be relevant to all the clients that the real time data server 502 iscommunicating with.

Process 1001 transmits datagrams from the real time data server 502 to aclient 511. Each packet includes an IP header, a UDP header and an MTPheader. For convenience each packet is referred to as a datagram.Process 1001 comprises two separate processes: datagram preparation 1002and output buffer processing 1003. Process 1002 prepares data fortransmission. Data received from application server 501 can be fromseveral applications having different data characteristics andpriorities and it must be processed before it can be sent to terminalssuch as PDA 511.

Process 1004 receives datagrams from client terminals such as PDA 511and comprises three separate processes: datagram reception 1005,transactional datagram processing 1006 and streamed datagram processing1007.

Process 1008, which will be described further with reference to FIG. 27,performs background processing, which includes various processesrequired to be performed while transmitting and receiving data, such asidentifying timeout conditions.

Process 1009 provides session maintenance, which includes operationsperformed when PDA 511 is temporarily disconnected. This process, whichwill be described further with reference to FIG. 30, is the first tostart, with processes 1001, 1004 and 1008 being performed once the usersession is established.

FIG. 11

FIG. 11 illustrates the structure of a typical datagram 1101 sentbetween the real time data server 502 and PDA 511. A local networkprotocol header 1102 changes as the datagram passes from network tonetwork across the Internet 110. An IP header 1103 defines thedestination of the packet, as well as other characteristics. A UDPheader 1104 precedes an MTP header 1105, which implements severalfeatures for efficiently supplying real time data to clients over mobilewireless links, as well as other data links of varying degrees ofquality. The MTP header 1105 is followed by data 1106 that has a maximumlength, in this embodiment, of approximately 500 bytes. This limit ischosen to avoid packet fragmentation and to avoid overloading theterminals, and could be varied.

The IP header 1103 includes several fields. Version field 1108 indicatesthe version of IP being used, for example IPv4 or IPv6. Internet HeaderLength field 1109 indicates the length, in 32-bit words, of the IPheader. Its minimum value is 5. Length field 1110 gives the totallength, in bytes, of the datagram, including the IP header (but notincluding the local network protocol header 1102). Protocol field 1111is set to a value indicating that UDP is being used. Source IP addressfield 1112 gives the return address of the datagram, while destinationIP address field 1113 gives its destination.

The UDP header 1104 has the following fields. Source port field 1114gives the port on the computer sending the datagram, while destinationport field 1115 gives the port number on the computer receiving thedatagram. Length field 1116 gives the length of the datagram in bytes,including the UDP header but not including the previous headers 1102 and1103. Checksum field 1117 contains a value computed from the IP header1103, UDP header 1104 and the remainder of the datagram, enabling dataintegrity to be confirmed.

FIG. 12

FIG. 12 details MTP header 1105. It contains a number of fields.Firstly, version number field 1201 gives the version of MTP being used.

Fields 1202 to 1209 are single-bit fields that are considered to be“set” if their value is one, and not set if it is zero. SYN field 1202and KAL field 1213 are used for signalling. At the start and end of asession, SYN field 1202 is used for handshaking, but it is also used toperform various connection timing procedures. KAL field 1213 is used tosend “keep alive” datagrams that indicate that a connection is open. ACKfield 1203 indicates that the datagram is being used to acknowledge areceived datagram, while EACK field 1204 indicates an extendedacknowledgement. STREAM field 1205 is used to differentiate betweenstreamed and transactional data. When set, it indicates that thedatagram contains streamed data.

START field 1206 and END field 1207 are used to indicate that a datagramcontains data and that it is the first or last of a set. If a datagramis too large to be sent as a single datagram then it may be split, andso START field 1206 indicates the first datagram and END field 1207indicates the last. A datagram that has not been split has both fieldsset. An empty datagram does not have these fields set.

RESET field 1208 is used for session handshaking when restarting asession, and FINISH field 1209 is used to close an MTP session.

Session ID field 1210 is a number indicating which session the MTPdatagram relates to. Sequence number field 1211 is a number indicatingthe datagram sequence. Each datagram that is sent out and that requiresacknowledgement is given its own effectively unique number, which isthen used in an acknowledgement by the client. (Since streamed andtransactional datagrams are numbered using a different sequence, andsince the sequence numbering loops at a number that is greater than thenumber of acknowledgements that will be outstanding at any time, thesequence number is not strictly unique but is effectively unique.) Anacknowledgement is itself a datagram, which may contain data, and soacknowledgement number field 1212 is the sequence number of the datagrambeing acknowledged in a datagram that has the ACK field 1203 set. Thisdatagram is probably otherwise unconnected with the datagram beingacknowledged.

FIG. 13

FIG. 13 details process 1002 at which datagrams are transmitted. Process1001 comprises two, effectively concurrent processes 1002 and 1003.Process 1002 fills up the transactional and streamed segment buffers 905and 906, while process 1003 looks in the buffers and marks the datagramsfor sending.

Process 1002 commences with step 1301 at which a question is asked as towhether there is any data for transmission. If this question is answeredin the affirmative then a further question is asked at step 1302 as towhether the data is transactional data. If this question is answered inthe affirmative then at step 1303 a datagram is prepared and at step1304 it is placed in the transactional segment buffer 905.Alternatively, if the question asked at step 1302 is answered in thenegative, a datagram of streamed data is prepared at step 1305. Theelapsed time value in the datagram is set to zero, indicating freshdata, at step 1306 and at step 1307 the datagram is placed in thestreamed segment buffer 906.

Following steps 1303 or 1307, or if the question asked at step 1301 isanswered in the affirmative, control is returned to step 1301 and thequestion is asked again as to whether there is any data fortransmission.

FIG. 14

FIG. 14 illustrates the process performed during steps 1303 to 1307, inwhich data is prepared for transmission. A datagram 1401 can comprisetransactional data or streamed data, which is determined by whether ornot STREAM field 1205 is set in the MTP header 1105. Each of the twotypes of data has its own buffer, transactional segment buffer 905 andstreamed segment buffer 906, from which datagrams are sent. Onceacknowledged, a datagram can be deleted from its location in segmentbuffer 905 or 906. Each segment buffer stores a number of datagrams.

Transmission is facilitated by supplying a datagram to the operatingsystem 801, which facilitates its electronic transmission using theInternet Protocol.

Transactional and streamed datagrams are generated from data stored inprioritised message queues 907. This data is supplied to message queues907 by applications running on application server 501. An applicationmay supply all its outgoing messages to a particular message queue, ormay pass messages to different queues depending upon the nature of thedata.

Transactional data is supplied to prioritised message queues 1402, 1403and 1404. Streamed data is supplied to prioritised message queues 1405,1406 and 1407. Each message queue may contain a number of messagessupplied from applications on application server 501. These messages aredelineated by level one message headers, such as header 1408, thatspecify the length of the data and the application from which it wassupplied.

The amount of data taken from each message queue and combined into asingle datagram depends upon proportions defined for each message queue.For example, default proportions of fifty percent, thirty percent andtwenty percent may be assigned to prioritised message queues 1405 to1407 respectively. If message queue 1407 has no data then its allocationwill be equally reallocated between queues 1406 and 1407, giving queue1408 thirty-five percent and queue 1407 sixty-five percent. If only onequeue contains data then it will have one hundred percent of theallocation.

The way the data is allocated also depends upon the type of messagequeue. Transactional messages may be broken up over a number ofdatagrams, and so the process only considers the amount of data in thequeue. However, streamed messages must be wholly contained within onedatagram, and so only entire messages are taken from these messagequeues, even if this means that the message queue's priority allocationis not used up.

Datagrams are created from the message queues and placed in segmentbuffers 905 and 906. These are then sent, with the first message beingtaken from each segment buffer in turn.

The example in FIG. 14 shows datagram 1401, which is made up fromtransactional data. The amount of data that can be included in thedatagram is calculated, and data is taken from each of queues 1402 to1404 according to their priority levels. Data from different prioritisedmessage queues is delineated within a datagram by level two messageheaders, such as headers 1409, 1410 and 1411. These headers include alength field 1412 and a message queue field 1413.

Thus the example datagram 1401 does not contain a single message but infact contains portions of five messages, since the data from each ofqueues 1402 to 1404 includes a message header and thus includes the endof one message and the beginning of another.

The number of prioritised message queues shown here and theirproportions are provided as an example only. There could be fewerqueues, for example only one transactional queue and two streamedqueues, or any other number. The proportions will vary according to thekinds of real time data provided and the realities of each individualsystem. Additionally, it is not necessary that unused allocation beequally divided between the remaining queues. It could be dividedaccording to their own allocations, or in some other way.

FIG. 15

FIG. 15 details step 1303, at which a transactional datagram isprepared. At step 1501 an MTP header is created in a temporary buffer.This is a default header that as yet does not contain any informationspecific to the datagram being considered. This information is added bybuffer processing process 1003, which will be described with referenceto FIG. 17. At step 1502 a variable N is set to be the number oftransactional prioritised message queues 1402 to 1404 that contain data,and a variable Y is initialised to zero. At step 1503 the number ofbytes available for data, indicated by variable S, is calculated bysubtracting the product of N and the level two header size from themaximum data size. For example, the maximum data size may be 500 bytes.

At step 1504 the variable N is decremented by one and at step 1505 thehighest message queue is selected. A variable P is set to be the sum ofthe proportion of the datagram that the data in that queue may use, forexample 0.3 for queue P1, and variable Y (zero on the first iteration),and a variable X is set to be the amount of data, in bytes, in thequeue. At step 1506 a question is asked as to whether the variable N isequal to zero. If this question is answered in the affirmative then thequeue under consideration is the last one containing data and so thefollowing steps need not be carried out, control being directed to step1513.

However, if it is answered in the negative then at step 1507 a furtherquestion is asked as to whether the variable X is less than the productof the variables S and P; that is, whether the amount of data in thequeue is less than the amount of data that may be used. If this questionis answered in the affirmative then at step 1508 the variable Y iscalculated as the variable X subtracted from the product of P and S, alldivided by the product of S and N, all added to the previous value of Y.Thus Y is a proportion that is to be added to the proportions of theremaining queues in order to allocate to them the unused space allocatedto the queue under consideration. For example, if the available space is400 bytes and all three queues contained data, then P1 is allocated 120bytes. If it only contained 100 bytes then a further 10 bytes would beallocated to each of the remaining queues. Y would thus be 0.05.Alternatively, if the question asked at step 1507 is answered in thenegative, to the effect that the variable X is not less than the productof X and S, then at step 1509 the variable X is set to be the product ofthe variables P and S.

Following either step 1508 or step 1509, or if the question asked atstep 1506 is answered in the affirmative, at step 1510 a level twoheader is created in the temporary buffer and the first X bytes aremoved from the queue into the temporary buffer. The question is thenasked at step 1511 as to whether the variable N is equal to zero. Ifthis question is answered in the negative then control is then returnedto step 1504 where N is decremented again before the next queue isselected. If it is answered in the affirmative then step 1303 is overand a datagram has been prepared. The step at 1304 of placing thisdatagram in the transactional segment buffer 905 consists of moving thedata from the temporary buffer to he transactional segment buffer 905.

FIG. 16

FIG. 16 details step 1305, at which a streamed datagram is prepared fromthe data in streamed prioritised message queues 1405 to 1407. At step1601 an MTP header is created in a temporary buffer, and at step 1602 avariable N is set to be the number of streamed message queues thatcontain data, while variables X and Y are set to be zero.

At step 1603 the available space S is calculated in the same way as atstep 1503, except that a further two bytes are subtracted, which will beused to store the elapsed time. At step 1604 the variable N isdecremented by one.

At step 1605 a level two header is created in the temporary buffer, andat step 1606 the first message queue is selected, and a variable P setto be the sum of the queue's priority proportion and the variable Y. Atstep 1607 the first message in the queue is selected, and the variable Xis set to be the sum of the message's length in bytes and the previousvalue of X. At step 1608 a question is asked as to whether the variableX is less than the product of the variables P and S.

If this question is answered in the affirmative then at step 1609 themessage is moved to the temporary buffer and a further question is askedas to whether there is more data in the queue. If the question isanswered in the negative then control is returned to step 1607 and thenext message is selected.

If the question asked at step 1608 is answered in the affirmative, orthe question asked at step 1610 is answered in the negative, then atstep 1611 the variable X is reset to zero, and the variable Y is updatedto be the previous value of the variable X subtracted from the productof P and S, all divided by the product of S and N, all added to theprevious value of Y. A question is then asked at step 1612 as to whetherN is equal to zero. If this question is answered in the negative thencontrol is returned to step 1604. If it is answered in the affirmativethen step 1605 is concluded.

Thus only entire messages are included in a streamed datagram, althoughmore than one message may be contained in a single datagram. A streameddatagram may contain more than one message from a single queue, as longas it does not exceed its priority allocation, but may not contain afragment of a datagram.

As discussed above, the algorithm presented in FIG. 15 and FIG. 16 isonly one possibility for prioritising data.

Additionally, in another embodiment (not shown) this prioritisation andmixing of data does not take place. In that embodiment datagrams onlycontain data from a single application, and the START field 1206 and ENDfield 1207 are used to indicate the beginning and end of messages,instead of the level one headers described with reference to FIG. 14.

FIG. 17

Output buffer processing 1003 is detailed in FIG. 17. At step 1701 aquestion is asked as to whether both the transactional segment buffer905 and the streamed segment buffer 906 are empty, and if this questionis answered in the negative then the next datagram to be sent in eitherbuffer 905 or 906 is marked for transmission (the process alternatesbetween the two buffers) at step 1702. This may be the next newestdatagram, or it may be an unacknowledged datagram that has been markedto be resent.

If the question asked at step 1701 is answered in the negative then atstep 1703 a further question is asked as to whether an acknowledgementis required. If this question is answered in the affirmative then atstep 1704 an empty acknowledgement datagram is created. If the questionasked at step 1703 is answered in the negative then at step 1705 afurther question is asked as to whether a heartbeat datagram isrequired, and if this question is answered in the affirmative then alatency-measuring datagram is produced at step 1706 (this will bedescribed more fully with reference to FIG. 20). If the question askedat step 1705 is also answered in the negative then control is returnedto step 1701 and the question is asked again as to whether the buffersare empty.

Following any of steps 1702, 1704 or 1706, the MTP header as describedin FIG. 12 is set at step 1707. At step 1708 the process waits for atransmission time, since the rate of datagram transmission iscontrolled, as will be described with reference to FIG. 29. When thistransmission time is reached, the time of sending is internally recordedfor the purposes of delaying the next transmission. It is recorded withthe datagram stored in the segment buffer, along with an indication ofhow many times the datagram has already been sent. At step 1710 aquestion is asked as to whether this datagram is being resent and isalso a datagram containing streamed data, as indicated by the setting ofboth STREAM field 1205 and START field 1206; if so the elapsed time ischanged at step 1711 to reflect the amount of time since the firstattempt at sending the datagram, as can be calculated from the time ofthe last sending and any previous value of the elapsed time. This is tofaciliate the calculation of resend latency, as will be described withreference to FIG. 41. Finally, at step 1712, the datagram is sent.

FIG. 18

FIG. 18 details step 1705, at which the MTP header is set. At step 1801a question is asked as to whether there is a datagram received from theclient that needs to be acknowledged. The answer to this questiondepends not only on whether a datagram has been received from PDA 511but also what kind of datagram it is. A datagram containingtransactional data is acknowledged immediately, as is any datagram beingused for timing purposes, and so if either of these have been receivedbut not acknowledged the question is answered in the affirmative.Streamed data, being less critical, is acknowledged using an extendedacknowledge, in which multiple packets are acknowledged in order tolower network traffic. Thus if only streamed datagrams have beenreceived then the question will be answered in the affirmative only if asuitable period of time has elapsed. Otherwise, or if no datagrams havebeen received at all, the question is answered in the negative.

If the question asked at step 1001 is answered in the affirmative thenat step 1802 ACK field 1203 is set and the sequence number of thedatagram being acknowledged is entered in acknowledgement number field1212. At step 1803 a question is asked as to whether thisacknowledgement is an extended acknowledgement. If this question isanswered in the affirmative then at step 1804 the EACK field 1204 isalso set, and any datagrams that have not been received but have lowersequence numbers than the sequence number contained in field 1212 arelisted as data in part 1106 of the datagram. Thus these datagrams arenegatively acknowledged. Since the IP header 1103 and UDP header 1104both contain length fields indicating the total length of the datagramthe recipient of an extended acknowledgement knows implicitly how manydatagrams are being negatively acknowledged. At this point, and if thequestion asked at step 1803 is answered in the negative, step 1705 iscompleted. (Note that because transactional datagrams have a separatesequence number from streamed datagrams, the extended acknowledgementprocess does not interfere with the acknowledgement of transactionaldatagrams.)

However, if the question asked at step 1801 is answered in the negative,to the effect that an acknowledgement is not due, at step 1805 a furtherquestion is asked as to whether a latency measurement or heartbeatshould be initiated. If this question is answered in the affirmativethen at step 1806 SYN field 1202 is set to one. A datagram having thisfield set initiates a latency measurement. When an acknowledgingdatagram is received from PDA 511 it is used to measure round-triplatency (further described with reference to FIG. 17). (Thus the SYNfield cannot be set in an acknowledging datagram. For this reason step1805 is only initiated if the question asked at step 1801 is answered inthe negative.) Alternatively, if no data is being sent, a datagramhaving this field set, in addition to being used to measure latency,provides a heartbeat that confirms that the connection is still open.

Following step 1806, or if the question asked at step 1805 is answeredin the negative, step 1705 is completed.

This figure highlights one of the few ways in which the server and theclient are not symmetrical. While a session is stalled, the server willnot send heartbeat datagrams, but the client will. This is because thereceipt of a datagram from the client by the server ends the stall. Thisis provided by the suspension of background processing process 1008,which makes the decision as to whether to send a heartbeat datagram,during a stalled session. However, process 1003 sends the datagram, ifinstructed to, in exactly the same way on both the server and theclient.

FIG. 19

FIG. 19 details process 1005 that receives datagrams sent by PDA 511. Atstep 1901 an incoming datagram is received and the receive time logged.A question is then asked at step 1902 as to whether the datagram has asequence number identical to a recently received datagram of the sametype (ie streamed or transactional). This can happen whenacknowledgements and resends “cross” and when acknowledgements are lostover the network. Thus if this question is answered in the affirmativethen control is directed to step 1912 and the datagram is acknowledgedwithout being processed. Alternatively, if it is answered in theaffirmative, then at step 1903 a question is asked as to whether the SYNfield 1202 is set, indicating that the datagram is a latency-measurementdatagram. Thus if this question is answered in the affirmative then atstep 1904 a further question is asked as to whether ACK field 1203 isalso set. If this question is also answered in the affirmative then thedatagram is a returned latency-measurement datagram and so the latencyis calculated at step 1905.

Alternatively, if it is answered in the negative, then at step 1906 aquestion is asked as to whether the acknowledgement number field 1212 iszero. If this question is answered in the affirmative then the ACK fieldis not set but an acknowledgement number is given. This indicates thatthe acknowledgement field does not contain a sequence number butindicates a new heartbeat rate, measured in milliseconds, and thus theheartbeat timing rate contained in the session data 804 is updated atstep 1907. This process will be described further with reference to FIG.42.

Following either of steps 1907 or 1905, or if either the question askedat step 1903 is answered in the negative or that asked at step 1906 isanswered in the affirmative, then control is directed to step 1908, atwhich a question is asked as to whether the datagram contains streameddata, as indicated by the setting of STREAM field 1205. If this questionis answered in the affirmative then the resend latency is recalculatedat step 1909. Resend latency, in combination with connection latency, isused to estimate the age of data received, and is described further withreference to FIG. 41.

Following this, or if the question asked at step 1908 is answered in thenegative, acknowledgements and state changes are processed at step 1910,as will be further described with reference to FIG. 21.

Finally the data 1106 is extracted at step 1911, as will be furtherdescribed with reference to FIG. 23 and FIG. 24 and the datagramacknowledged at step 1912. The processing steps 1901 to 1910 relate onlyto the information contained within the MTP header 1105, much of whichis not connected with the data in any way.

FIG. 20

FIG. 20 illustrates the use of the SYN field 1202 to measure connectionlatency. It is necessary that at all times the client terminals areaware of exactly how old the data is. This is not possible usingtraditional methods such as, for example, clock synchronisation, becausethere may be thousands of terminals. Thus the system described hereinprovides a method of measuring the connection latency between the RTDP101 and each of its terminals.

A latency-measurement datagram is sent at regular intervals by settingthe SYN field 1202 in an outgoing datagram in either transactionalsegment buffer 905 or streamed segment buffer 906 and noting the time atwhich it was sent. As an example, transactional segment buffer 905 isshown, containing several packets 2001, 2002 and 2003. The questionasked at step 1805 is answered in the affirmative, to the effect that alatency measurement should be initiated, and so the SYN field 1202 ofthe next datagram to be sent, which is datagram 2001, is set.

Datagram 2001 takes a number of milliseconds, shown by arrow 2004 andidentified by the variable A, to be transmitted to PDA 511, whosereceive buffer 2005 is shown. A process running on PDA 511, which issubstantially identical to process 1005, sets the SYN field 1202 and theACK field 1203 in its next outgoing datagram 2006. This process takes atime indicated by arrow 2007 and identified by the variable B. Finally,transmission of datagram 2006 back to real time data server 502 takes atime indicated by arrow 2008 and identified by the variable C. Whendatagram 2002 is received at real time data server 502 the fact thatboth the SYN and ACK fields are set triggers latency calculation at step1905.

The round trip time, which is obtained by comparing the receive time ofdatagram 2002 with the transmission time of datagram 2001, is equal tothe sum of the variables A, B and C. Since network conditions are, onaverage, symmetric, A is assumed to be approximately equal to C. B isvery small because it is possible to directly acknowledge packet 2001without waiting for any out-of-order datagrams that would have to bereceived if the latency was measured using a cumulative acknowledgement,as with TCP. Thus, as shown by equation 2006, the two-way latency isapproximately equal to the round trip time, and the one-way latency, orconnection latency, is half the round trip time.

Having obtained a value for the round trip time, it is filtered usingequations 2007. K is an adaptive filter coefficient that is varied inorder to optimise the ability of the filtered latency to follow quickchanges when these are consistently observed. Thus the filtered latencyis equal to the sum of the following factors: K subtracted from one allmultiplied by the measured latency; and K multiplied by the previousfiltered latency calculation. Other filtering or weighting methods maybe used in order to smooth the variability of the latency calculation.

The round trip time is used by both the server and the client todetermine the length of time that should be waited for anacknowledgement before a transactional datagram is resent (timeout).Since streamed datagrams may be acknowledged using an extendedacknowledgement, the time that a process waits before sending anextended acknowledgement is added to the latency value to produce thetimeout for streamed datagrams. The constant measurement of the latencydescribed above ensures that the timeout settings are as accurate aspossible. A fixed timeout setting can be set too high, in which case thewait before resend would be too long, thus degrading the timeliness ofthe data, or it can be too low, resulting in too many resends. Thisdynamic timeout creates a compromise.

The round trip time may be halved to give a connection latency, whichindicates the approximate time taken by a datagram to be sent from theserver to the client. This value is used by the client to indicate thetimeliness of received data, and will therefore be described furtherwith reference to FIG. 41. Resend latency measurement, which will bedescribed with reference to FIG. 41, is also calculated at both theclient and the server end but in this embodiment is only used by theclient. It will therefore not be discussed at this stage.

FIG. 21

FIG. 21 details step 1910 at which acknowledgements and state changesare processed. At step 2101 a question is asked as to whether the RESETfield 1208 or FINISH field 1209 (as contained in the MTP header 1105 ofthe datagram received at step 1901) is set, indicating that the sessionshould be reset or ended. If this question is answered in theaffirmative then a disconnect takes place at step 2102. This concludesstep 1910 if this route is taken.

If the question asked at step 2101 is answered in the negative then atstep 2103 a question is asked as to whether EACK field 1204 is set,indicating that the datagram contains an extended acknowledge. If thisquestion is answered in the affirmative then at step 2104 the extendedacknowledgement is processed. If it is answered in the negative then atstep 2105 a further question is asked as to whether ACK field 1203 isset, indicating that the datagram contains an acknowledgement. If thisquestion is answered in the affirmative then at step 2106 theacknowledgement is processed by removing the datagram that has thesequence number contained in SEQUENCE NUMBER field 1211 from therelevant segment buffer 905 or 906. If it is answered in the negative,or following step 2104, the session state variables 902 for PDA 511 aremodified if necessary.

FIG. 22

FIG. 22 details step 2104, at which an extended acknowledgement isprocessed. As described previously with reference to step 1804, anextended acknowledgement is in the form of a datagram with EACK field1204 set, a streamed datagram sequence number contained inacknowledgement number field 1212, and possibly a list of streameddatagram sequence numbers that have not been received by PDA 511 as data1106. Thus the process has a range of datagram sequence numbers toconsider. This range starts at the number following the sequence numbercontained in the last extended acknowledgement and finishes at thenumber contained in the extended acknowledgement currently beingconsidered.

Thus at step 2201 the first sequence number in this range is selected.At step 2202 the streamed datagram corresponding to this sequence numberis identified and at step 2203 a question is asked as to whether thesequence number identified at step 2201 is in the list of negativelyacknowledged datagrams contained in data 1106 of the datagram. If thequestion is answered in the negative then the sequence number is beingacknowledged and this is processed at step 2205. If the question isanswered in the affirmative then, since the identified datagram is stillstored in its relevant segment buffer 905 or 906, it is marked to beresent at step 2204.

At step 2206 a question is asked as to whether the sequence number beingconsidered is the same as the number contained in the acknowledgementnumber field 1212 of the datagram. If this question is answered in thenegative then control is returned to step 2201 and the next sequencenumber is selected. If, however, it is answered in the affirmative, thenthe extended acknowledgement has been fully processed and step 2104 iscompleted.

FIG. 23

FIG. 23 illustrates the reception of a datagram from PDA 511. A receivebuffer is provided by the operating system 801, which supplies adatagram to receiving transactional segment buffer 908 or to process1007, via process 1005. Once datagrams are ordered within transactionalsegment buffer 908, process 1006 decodes the level two message headersin the datagrams to split the data up and place it in the correct one ofprioritised message queues 910. There are three transactional queues2301, 2302, and 2303, corresponding to the message queues 1402 to 1404.Process 1007 performs the same function for streamed datagrams. There isno streamed segment buffer for incoming datagrams because there is noordering necessary. There are three streamed queues 2304, 2305 and 2306.These correspond to the prioritised message queues 1405 to 1407. Oncethe data is placed in the queues, level one headers indicate to theapplications that a message is complete and can be used.

In the alternative embodiment described with reference to FIG. 16,wherein mixing of the data is not used, only the transactional data needbe ordered in the segment buffer 908. In this embodiment every streamedmessage is contained in a single datagram and so need not be ordered. Ifan old update arrives after a new one then the old one is simplydiscarded by the relevant application. However transactional data, whichusually stretches over more than one datagram, must be ordered. TheSTART and END fields 1206 and 1207 are used to indicate when a messageis finished and can be transferred to the relevant message queue.

FIG. 24

FIG. 24 details step 1911, at which the data contained in a receiveddatagram is extracted and acknowledged. At step 2401 a question is askedas to whether the received datagram contains data in portion 1106. Ifthis question is answered in the negative then a further question isasked at step 2402 as to whether STREAM field 1205 is set, indicatingthat the datagram contains streamed data. If this question is answeredin the negative then at step 2403 the data is placed in transactionalsegment buffer 908, while if it is answered in the affirmative then thedata is passed to process 1007 at step 2404.

Following step 2404, or if the question asked at step 2401 is answeredin the negative, to the effect that the datagram contains no data, thena question is asked at step 2405 as to whether SYN field 1202 is set,indicating that the datagram is a latency measurement or heartbeatdatagram. If this question is answered in the affirmative, or followingstep 2403, the datagram is immediately acknowledged at step 2406. Thisstep involves flagging the sequence number in order that process 1003acknowledges it in the next available outgoing datagram at step 1707 asdescribed with reference to FIG. 18. (If there is no outgoing datagram,then an empty streamed datagram is created.) At this point, or if thequestion is answered in the negative, step 1911 is concluded. Thustransactional and latency-measurement datagrams are acknowledgedimmediately. Streamed datagrams are acknowledged using an extendedacknowledgement, and empty datagrams that are not tagged, for example anacknowledgement containing no data, are not themselves acknowledged.

FIG. 25

FIG. 25 details process 1006 which processes the datagrams placed in thetransactional segment buffer 908. At step 2501 a question is asked as towhether there is data in the transactional segment buffer, and if thisis answered in the negative then the question is asked again until it isanswered in the affirmative, when at step 2502 a question is asked as towhether the first datagram in the segment buffer has the next expectedsequence number and is complete (as described with reference to FIG. 12,a datagram can be split over more than one datagram, and if this happensthen the full set of datagrams must be received before they can beprocessed). If this question is answered in the affirmative then thedatagram can be processed, and at step 2504 the first level two messageheader in the datagram is read to obtain the length of the datafollowing it and the message queue into which it is to be placed. Theindicated amount of data is then removed from the segment buffer andplaced in the correct queue at step 2505, with the level two header andMTP header being discarded. At step 2506 a question is asked as towhether there is another level two header, and if this question isanswered in the affirmative then control is returned to step 2504. If itis answered in the negative, or if the question asked at step 2503 isanswered in the negative, to the effect that the next datagram insegment buffer 908 is not the next expected one, control is returned tostep 2501 and the process waits for more data.

FIG. 26

FIG. 26 details process 1007, which processes incoming streameddatagrams. Since streamed datagrams do not have to be ordered, there isno necessity for an incoming streamed segment buffer. Thus at step 2601a streamed datagram is received from process 1005, and at step 2602 thefirst level two message header in the datagram is read to obtain thelength of the data following it and the message queue into which it isto be placed. The indicated amount of data is then removed from thedatagram and placed in the correct queue at step 2603, with the leveltwo header and MTP header being discarded. At step 2604 a question isasked as to whether there is another level two header, and if thisquestion is answered in the affirmative then control is returned to step2602. If it is answered in the negative, control is returned to step2601 and the process waits for more data.

FIG. 27

FIG. 27 details background processing process 1008. (This process issuspended on the server if the session is stalled. It is never suspendedon the client.) At step 2701 the process considers whether or not alatency-measurement datagram needs to be sent. If so, a flag is setwhich triggers the question asked at step 1805, as to whether such adatagram should be sent, to be answered in the affirmative. It alsotriggers the question asked at step 1702 as to whether a heartbeatdatagram is needed, which is asked only if both segment buffers areempty, to be answered in the affirmative. Thus if there is an outgoingdatagram at the point where a latency-measurement datagram is required,then that datagram has its SYN field 1202 set. However, if there is nooutgoing datagram then process 1003 creates one at step 1703. This isreferred to as a heartbeat, but it is also a latency-measurementdatagram. (It is also possible to use the KAL field 1213 as a heartbeat.A datagram with this field set is not acknowledged and not used as alatency-measuring datagram, but merely indicates that the connection isopen.)

At step 2702 the process negotiates a new heartbeat rate, if required.This is the maximum interval that should pass without data being sent oneither the server or client side. If no data is sent, then a heartbeatdatagram, which is an empty streamed datagram with the SYN field 1202set, is sent. The server does not send heartbeats during stalling of asession. This is achieved by the suspension of process 1008 when asession is stalled. The negotiation of a heartbeat rate, althoughavailable to both client and server, is in this embodiment predominantlyinitiated by the client and will therefore be described with referenceto FIG. 42.

At step 2703 the process flags the necessity for an extendedacknowledgement, if one is due, which leads to the question asked byprocess 1003 at step 1803 being answered in the affirmative. At step2704 the process marks for resending any datagrams that have not beenacknowledged within a timeout, and are thus still within theirrespective segment buffer 905 or 906. This is done by flagging thedatagram for resending, and it also increments the value in resend field1119 by one, to indicate the number of times the datagram has beenresent.

At step 2705 the process updates the timeouts based on connectioncharacteristics. The timeout for a transactional datagram is equal to(or slightly larger than) the round trip time calculated at step 1905.The timeout for a streamed datagram is equal to (or slightly largerthan) the round trip time calculated at step 1905 plus the time that theprocess will wait before sending an extended acknowledgement.

At step 2706 the process recalculates the data transmission rate, ifnecessary. This recalculation is done at specified intervals, and thusmay not be carried out on every cycle.

At step 2707 the process sends an update of network characteristics tothe application server, for use by the applications. In this embodimentthis update includes the amount of data currently being sent per second(in datagrams or in bytes), the amount of data in the segment bufferthat has the most data, or alternatively in both segment buffers, andthe round trip time; in other embodiments the update could include moreor less information.

Control is then returned to step 2701 and the process cycles untilterminated.

FIG. 28

FIG. 28 illustrates an extended acknowledgement. The MTP header 1105 anddata 1106 of a datagram are shown. In the header 1105 the EACK field1204 is set. The acknowledgement number field 1212 contains the sequencenumber of the most recent streamed datagram received. The data portion1106 contains a list of sequence numbers 2801, 2802 and 2803 that arelower than the number contained in field 1212 but which have not beenreceived. The datagrams corresponding to these numbers are thereforenegatively acknowledged.

FIG. 28A

FIG. 28A illustrates two of the different ways in which transactionaland streamed data is treated. The word data is herein applied to allkinds of data; including the information received from feeds 503 to 507,the messages containing the information produced by application server501 and the datagrams that contain a part or whole of these messagesproduced by real time data server 502, the messages received by aterminal and the information displayed by that terminal to the user.

Application server 501, part of real time data provider 101, producestransactional messages 2811, 2812, 2813 and 2814 and streamed messages2815, 2816, 2817 and 2818. Process 1102 on real time data server 502sends these messages to a terminal such as PDA 511 in the form ofdatagrams. Transactional messages 2811 to 2814 are split and sent aspart of datagram 2819, 2820 and 2821. For example, datagram 2819 mayconsist of a part of message 2811, a part of message 2812 and a part ofmessage 2814. Streamed messages 2815 to 2818 are not split. Thusdatagram 2825 consists of the whole of messages 2815 and 2816. Datagram2826 consists of message 2817. The whole of message 2818 cannot also fitinto the datagram, and so it is sent even though it is not at themaximum size. Datagram 2827 contains message 2818. Thus transactionalmessages may be split over at least two datagrams, while streamedmessages must be contained within a datagram.

Another difference in the treatment of transactional and streamed datais the method of acknowledgement. Thus each of transactional datagrams2819 to 2821 is individually acknowledged using acknowledgements 2822,2823 and 2824. However, streamed datagrams 2825 to 2827 may beacknowledged by PDA 511 using a single extended acknowledgement 2828,unless they are control datagrams that have a field such as SYN 1202,RESET 1208 or FINISH 1209 set, in which case they are individuallyacknowledged.

FIG. 29

FIG. 29 details step 2706, at which the data transmission rate iscalculated by updating the transmission interval (the time that process1003 waits after sending a datagram before sending another datagram).Although each of the streamed and transactional data being sent from theRTDP 101 to each of its clients is relatively small in data terms, itmust be provided in a timely fashion. Congestion should therefore beavoided. Existing protocols such as TCP merely react to congestionrather than preventing it, and as shown in FIG. 4 have a very slowrestart when a connection is cut off.

This problem is solved by having a separate transmission rate for eachterminal, and constantly monitoring each of these rates to keep itoptimum. Thus at step 2901 a question is asked as to whether theinterval since the last update is less than the product of 1.25 and theround trip time calculated at step 1905. If this question is answered inthe negative then it is not yet time to perform the calculation and step2706 is concluded. This is because the effect of a previous update tothe transmission rate is not felt until at least one round trip timelater, and thus the calculation interval is a small amount more than theround trip time—a quarter of the round trip time in this embodiment.

However, if the question is answered in the affirmative then at step2902 the total number of resends in the streamed segment buffer 906 isdetermined and set as the value of a variable R. The number of resendsis a sum of the number of datagrams in the buffer that are tagged to beresent, with an indication that a datagram is on its second resendadding two to the total, an indication that a datagram is on its thirdresent adding three to the total, and so on.

At step 2903 a question is asked as to whether the value of R is zero,meaning that there are no datagrams in the buffer that are being resent.This indicates that the rate of transmission can be increased. Thus ifthis question is answered in the affirmative then a further question isasked at step 2904 as to whether the current interval betweentransmissions is significantly larger than a value saved as the “currentbest interval”. If this question is answered in the affirmative then thetransmission interval is decreased by a first, larger amount at step2905, while if it is answered in the negative then the transmissioninterval is decreased by a second, smaller amount at step 2906. Thismeans that when the transmission interval is much larger than the lastknown achievable interval, the transmission interval is decreased muchfaster than when it is close to it.

If the question asked at step 2903 is answered in the negative, to theeffect that R is not zero, then at step 2907 a question is asked as towhether R is less than a certain threshold. If this question is answeredin the affirmative then the transmission rate is not changed. If,however, it is answered in the negative then a further question is askedat step 2908 as to whether R is significantly smaller than the previousvalue of R. If this question is answered in the affirmative then therate is not altered, even though R is above the threshold, because thisvalue of R may be an anomaly.

If R is above the threshold and not significantly smaller than theprevious R, then this indicates that there are too many resends and theinterval between datagram transmissions needs to be increased. However,first a question is asked at step 2909 as to whether the last change inthe interval was a decrease. If this question is answered in theaffirmative then the current transmission interval is the lowest knownachievable interval at the current time, and so it is saved as thecurrent best at step 2910. The transmission interval is then increasedat step 2911 (the step size used in this embodiment is larger than bothof the step sizes used for decreasing the transmission interval).

The algorithm described herein is a robust method of attempting toincrease the rate of datagram transmissions while minimising the numberof resends, using continual and rapid adjustment. It provides a quickresponse to decreases in the available network bandwidth and a fastrestart when transmission is temporarily cut off or after a congestionspike. Clearly the implementation details of the algorithm, such as thenumber of step sizes and what is meant by “significantly large” could bechanged.

In this embodiment, due to the small receive buffer of PDA 511, it isonly possible to send one datagram at a time. However, in otherembodiments, the method could be altered by sending more than onedatagram at once when the transmission interval reaches a certain lowthreshold. It can be more efficient to send two packets at once at alarger interval than to continue decreasing the transmission interval.

Additionally, in another embodiment it could be the transactionalsegment buffer or both segment buffers that are considered when summingthe resends.

FIG. 30

FIG. 30 details process 1009, which performs session maintenance. Thisprocess notes certain information available in the headers of datagramsas they arrive and maintains the client sessions accordingly, but doesnot interfere with the processing of the datagrams. Thus at step 3001 adatagram is received, and at step 3002 a question is asked as to whetherthe datagram header contains valid session details, for example sessionnumber, encryption and so on.

If this question is answered in the negative, meaning either that thedatagram has no session number or that it contains invalid sessiondetails, then at step 3003 a further question is asked as to whether thedatagram is requesting a new session, indicated by the lack of a sessionnumber and the setting of SYN field 1202. If this question is answeredin the affirmative then at step 3004 a new session is created for theclient that sent the datagram. This includes creating session data 803and validating the new session, ie checking whether a valid accountnumber for an active account, valid name and correct password have beensupplied, and is in practice performed by calling a subroutine onapplication server 501, on which the user details are stored.

An answer in the negative to the question asked at step 3003 means thatthere is a problem of some kind with the datagram, for example itrelates to a terminated session or the session details do not match, andso the session is ended at step 3012 by sending a reset datagram (adatagram in which the RESET field 1108 is set) to the originating IPaddress and removing the session data, if there is any.

If the question asked at step 3002 is answered in the affirmative, tothe effect that the session details are valid, then a further questionis asked at step 3005 as to whether the IP address from which thedatagram was sent matches the IP address held in the session variables.If this question is answered in the negative then at step 3006 the IPaddress is updated in the session variables. The client could change IPaddresses for a number of reasons. The main ones are that a client thathas moved between networks or cells, thus changing its mobile IPaddress, or that a client deliberately terminated its IP connection inorder to fully use bandwidth for another function, for example to make atelephone call. In this case the client would probably be assigned adifferent IP address on reconnection, even if it is in the same cell ofthe same network. However, this functionality of MTP allows the clientto immediately restore the session without visible delay to the user.

At step 3007 a question is asked as to whether the datagram isterminating the session, indicated by a setting of FINISH field 1209. Ifthis question is answered in the affirmative then the session is endedat step 3012, but if it is answered in the negative then at step 3008 aquestion is asked as to whether another datagram has been received forthis session within two timeouts and if is answered in the affirmativethen control is returned to step 3001. This timeout is different fromthe resend timeouts discussed with reference to FIG. 27, and is set bythe heartbeat rate. The heartbeat rate is the maximum interval whichshould pass without receiving data from a client.

Thus, if the question is answered in the affirmative, indicating thatsince the receipt of the last datagram a period of time equal to twotimeouts has passed with no further communication from the client, thenat step 3009 the session is placed in a stalled state. This involvesnoting in the session variables that the session is stalled, whichprevents any more datagrams from being sent to the client. In thisembodiment, this involves suspending datagram reception process 1104 andbackground processing process 1109. A stalled session can occur becausethe network connection to the client has been broken, because the PDA511 does not currently require the real time data and has thereforestopped communicating, because the PDA 511 has been switched off withoutending the session, and so on.

At step 3010 a question is asked as to whether a datagram has beenreceived for this session within ten minutes of the session being placedin a stalled state, and if this question is answered in the affirmativethen the stall is ended and control is returned to step 3001. Ending astall involves changing the session state and restarting any suspendedprocesses. This will then have the effect of resending any datagramsthat have not been acknowledged. However, in an alternative embodimentthe streamed data buffer 906, and possibly the streamed message queues1405 to 1407, could be flushed on the ending of a stall.

If, however, the question asked at step 3010 is answered in the negativethen at step 3012 the session is ended. The session is closed after along stall firstly for security measures, because the user may have lefthis terminal unattended, and secondly to prevent memory space being usedfor an unwanted session, for example if the terminal has been switchedoff.

Stalling as described above solves the problem with spoofing—that onreconnection the telecoms gateway sends a large amount of data all atonce to the terminal, thus negating any value obtained by managing datatransmission rate as described with reference to FIG. 29. Instead, whenthe connection is broken and the real time data server 502 stopsreceiving datagrams from PDA 511 the session is stalled and the realtime data server 502 sends no more datagrams. Thus the telecoms gatewaybuilds up a very small amount of data, if any, to send on when theconnection is re-established.

The second problem solved here is the maintenance of a session when thePDA 511 moves between cells in a telecoms network or indeed betweennetworks. As soon as an incoming datagram that has the correct sessionID and encryption but a different IP address is received, the IP addressin the session data 804 is immediately updated so that datagramsintended for PDA 511 are sent to that IP address. The user thereforeperceives very little, if any, delay when moving between IP addresses.

FIG. 30A

The updating of IP addresses described with respect to step 3006 isillustrated in FIG. 30A. A session is described by its session data 804stored on application server 501. It includes a session ID field 901containing a session ID 3020 and an IP address field 3021 containing anIP address 3022. The session may be in an active state or may move to astalled state, as shown by arrow 3023, when no communication is receivedfrom the client within two timeouts as set by the heartbeat rate.

A datagram 3024 is received by real time data server 502. It includes asource IP address field 1112 in its IP header 1103 and a session IDfield 1210 in its MTP header 1210. The session ID 3020 matches thesession ID in field 901. However, the IP address 3025 does not match theIP address 3021 in the IP header 3022. The session data 804 is thereforeupdated immediately by replacing the IP address in field 3021 with IPaddress 3025. All datagrams produced are now sent to this new address.Receipt of datagram 3024 also ends any stall, if one existed, and so thesession is shown as active.

Thus there is herein described a method of communicating between aserver, such as real time data provider 101, and a terminal such as PDA511. PDA 511 establishes a first communication path with RTDP 101 servervia a first allocated address and RTDP 101 allocates an identificationreference, such as session ID 901, to a communication session uponinitiation of the communication session. During the communicationsession the first communication path is closed and replaced by a secondcommunication path via a second allocated address. RTDP 101 maintainsthe communication session after replacement of the first communicationpath with the second communication path by identifying theidentification reference and communicating with PDA 511 via the secondallocated address.

FIG. 31

FIG. 31 details application server 501. It comprises a centralprocessing unit (CPU) 3101 having a clock frequency of 3 GHz, a mainmemory 3102 comprising 2 GB of dynamic RAM and local storage 3103provided by a 130 GB disk array. A CD-ROM disk drive 3104 allowsinstructions to be loaded onto local storage 3103 from a CD-ROM 3105. AGigabit Ethernet card 3106 facilitates intranet connection to the realtime data server 502 and the feeds 503 to 507.

FIG. 32

FIG. 32 details steps carried out by application server 501. At step3201 the application server 501 is switched on and at step 3202 aquestion is asked as to whether the necessary instructions are alreadyinstalled. If this question is answered in the negative then at step3203 a further question is asked as to whether the instructions shouldbe loaded from the intranet. If this question is answered in theaffirmative then at step 3204 the instructions are downloaded from anetwork 3205. If it is answered in the negative then at step 3206 theinstructions are loaded from a CD-ROM 3207.

Following either of steps 3204 or 3206 the instructions are installed atstep 3208. At this point, or if the question asked at step 3202 isanswered in the negative, the instructions are executed at step 3209. Atstep 3210 the application server is switched off. In practice this willhappen very infrequently, for example for maintenance.

FIG. 33

FIG. 33 details the contents of memory 3002 during the running ofapplication server 501. An operating system 3301 provides operatingsystem instructions for common system tasks and device abstraction. TheWindows™ XP™ operating system is used. Alternatively, a Macintosh™,Unix™ or Linux™ operating system provides similar functionality.Application server instructions 3302 include an application manager 3303and applications 3304, 3305, 3306, 3307, 3308 and 3309, including anapplication for each of data feeds 503 to 507. Application data 3310 isdata used by the applications 3304 to 3309 and user account data 3311comprises details of users' accounts, including the validation datainformation required when starting a session. Live data feed buffers3312 are buffers for feeds 503 to 507. Other data includes data used bythe operating system and application server instructions.

FIG. 34

FIG. 34 details the instructions executed by application manager 3303 atstep 3209. At step 3401 a client logs on successfully to start asession, and at step 3402 the user's application requirements, as storedin his account, are noted. These include the exact data in which theuser is interested, for example stocks updates and news stories. At step3403 a default level of service is selected, which is also retrievedfrom the user account. Levels of service will be discussed further withreference to FIG. 36.

At step 3404 the application server 501 communicates with the client viareal time data server 502 by sending messages. The content of thesemessages is determined by the user's application requirements and thecurrent level of service.

At step 3405 a question is asked as to whether the session is stalled,which will be indicated to the application server 501 by real time dataserver 502, and if this question is answered in the affirmative then atstep 3406 a question is asked as to whether the stall has ended. If thisquestion is answered in the affirmative then at step 3407 a selectiveupdate of data is performed and control is returned to step 3404. Whilethe session is stalled, the application server 501 does not send anymessages to real time data server 502.

If either of the questions asked at steps 3405 or 3406 is answered inthe negative, to the effect that the session is not stalled or that thestall has not ended, then at step 3408 a further question is asked as towhether the session has ended. If this question is answered in theaffirmative then the session ends at step 3411. If, however, it isanswered in the negative then at step 3409 any change in applicationrequirements received from the client via real time data server 502 isprocessed, and at step 3410 any received network conditions update isprocessed to change the level of service, if necessary. Control is thenreturned to step 3404.

Although this process is described here in terms of a single client andsession, the skilled user will appreciate that step 3209 involvesapplication server 501 performing these steps for every session.

FIG. 35

FIG. 35 details the selective update performed at step 3407. At step3501 all waiting transactional messages are sent. At step 3502 thewaiting streamed messages are examined to identify messages that relateto the same data. If any are found, then the older ones are deleted.This means that if during a stall two or more updates have been producedfor the same data, as is particularly likely with stock prices, thenonly the newest update is sent. At step 3503 concatenation of messagesis performed if possible. This means that updates for data that have thesame priority level could be amalgamated into one message, instead ofbeing sent as individual messages. Finally, at step 3504, the streamedmessages are sent.

Thus, on a selective update, transactional messages are all sent,whereas only the newest streamed data is sent in order to avoidoverloading the network and the client.

FIG. 36

As described with respect to FIG. 27, the real time data server 502periodically supplies to application server 501 updates of certainnetwork condition indicators, which in this example comprise the currenteffective bandwidth, given by the amount of data being sent per second,the amount of data in one or more buffers, and the current round triptime. (In this sense, network includes the real time data server and theclient, as well as the Internet, mobile telephony network or LAN, or anyother networks in between.) The values of these indicators provide tothe application server 502 information regarding the amount of data thatcan be sent to this client. The applications 3304 to 3309 then use thisinformation to determine how much information of what type should besent and at what speed.

FIG. 36 thus illustrates different ways in which the level of servicecan be changed. Graph 3601 shows how a news application suppliesdifferent information dependent upon the effective bandwidth measurementsupplied. When the effective bandwidth is low, then only news headlinesare supplied. More effective bandwidth allows news summaries to besupplied, while even more allows limited graphics. When the effectivebandwidth is very high, the full stories are sent. This is an example ofhow the level of service sets the type of data sent.

Graph 3602 shows how to stock prices application could increase theinterval between sending messages as the amount of data in the buffersincreases. The application can then supersede data that is waiting to besent with a newer update to the same data, and amalgamate messages ifnecessary. This is an example of how the level of service sets theamount of data sent.

Graph 3603 shows how an exchange rate application could stop sendingupdates altogether if the connection latency is too high, send only someexchange rates if the connection latency is about normal, and send allthe rates that the user is interested in if the latency gets very low.This could be valuable if the user has indicated that he does not wantto trade on, and is therefore not interested in, certain exchange ratesif the latency is known to be above a certain threshold. This is anexample of how the amount and type of data sent could be set by thelevel of service.

These graphs are only examples of ways in which network conditionindicators could be used to vary the level of service. The exact way inwhich the level of service varies depends upon the applicationrequirements of the user, the particular type of application, the datathat the application supplies, and so on. Also, although these graphsindicate thresholds and linear correlations, the network conditionscould be used so that an increase or decrease in a value triggers anincrease or decrease in level of service, such that a particular valuedoes not necessarily indicate a particular level of service. The valuesof two or more network condition indicators could be combined toindicate whether the level of service should increase or decrease.Additionally, the application manager 3303 could make the necessity toconsider network conditions more binding on some applications thanothers.

Thus MTP provides an additional advantage over other protocols. Becauseof its management of transmission rate, as described with reference toFIG. 29, networks with high bandwidth and low latency are used just aseffectively as those with low bandwidth and high latency, but if networkconditions are better then more information is sent. Thus if, forexample, the user of PDA 511 moves into transmission range of WiFigateway 118 and the PDA detects this and starts using WiFi instead of atelecoms network, not only does the session maintenance described withreference to FIG. 30 enable the session to be continued seamlessly overthe higher capacity network, but the user may immediately perceive ahigher level of service, depending upon the application being used. Thusthe protocol makes the best possible use of low bandwidth and highlatency connections, but also provides high bandwidth, low latency userswith a high level of service and perceived functionality.

FIG. 37

FIG. 37 details PDA 511. As described above, this is an example of aterminal that could be used in a system embodying the invention. Itincludes a CPU 3701 with a clock speed of 370 megahertz (MHz) withmemory 3702 being provided by 64 megabytes (MB) of RAM. 256 MB ofnon-volatile FLASH memory 3703 is provided for program and data storage.Liquid crystal display 3704 is used to display information to the user.Input/output 3705 processes the input of the keys and buttons 513 whileaudio input/output 3706 provides a microphone and speaker interface foruse with the telephone facility. Universal Serial Bus (USB) input/output3707 is used to connect PDA 511 to another computer, or to the Internet110 via a wired connection. GPRS/WiFi connection 3708 and GSM connection3709 enable PDA 511 to connect to wireless networks, while Ethernet card3710 enables PDA 511 to connect to a wired network, for example via adocking station on a computer.

FIG. 38

FIG. 38 details steps carried out by PDA 511. At step 3801 PDA 511 isswitched on and at step 3802 a question is asked as to whether the realtime application instructions are already installed. If this question isanswered in the negative then at step 3803 the instructions aredownloaded from a network 3804. The instructions are then installed atstep 3805.

At this point, or if the question asked at step 3802 is answered in thenegative, the instructions are executed at step 3806. Instructions forother applications on PDA 511 are executed at step 3807. At step 3808the PDA is switched off.

FIG. 39

FIG. 39 details the contents of memory 3702 during step 3806. Anoperating system 3901 provides operating system instructions for commonsystem tasks and device abstraction. The Windows™ CE™ operating systemis used, but a different PDA-suitable operating system could be used.Data transport instructions 3902, substantially like those described forthe real time data server 502 except that there is only a singlesession, include MTP instructions. Real time application instructions3903 include individual real time applications such as financial dataapplication 3904. Application 3904 takes information provided viadatagrams into a message queue and displays it on display 3704 accordingto its interface and user setups. For example, it may provide stocksprices in a grid with news headlines scrolling along the bottom.

Web browser instructions 3905 and email client instructions 3905 areprovided. These applications could also use. MTP to communicate via thereal time application provider 101. RTDP 101 can forward informationfrom and to a third party using TCP and from and to a terminal usingMTP. This emphasises that the protocol described herein for providingreal time data could be used for communication of many types.

Session data includes segment buffers, priority buffer and statevariables as shown for session data 804 in FIG. 9. Real time applicationdata 3908 is data used by the application instructions 3903 and useraccount data 3909 comprises the user's password, name, billing detailsand so on. Other data includes data used by the operating system andother applications.

FIG. 40

Since MTP is a substantially symmetrical protocol there is no need todescribe in detail much of the real time application instructionsexecuted at step 3806. Datagrams are produced, transmitted and receivedin substantially the same way as the processes described with referenceto FIG. 10. Thus, as shown in FIG. 40, step 3806 where the client runsthe application instructions comprises the following processes runningsubstantially in parallel. Process 4001 transmits datagrams from theclient 511 to the real time data server 502. It comprises two separateprocesses: datagram preparation 4002 and output buffer processing 4003.Processes 4002 and 4003 are substantially identical to processes 1002and 1003 respectively.

Process 4004 receives datagrams from the real time data server 502 andcomprises three separate processes: datagram reception 4005,transactional datagram processing 4006 and streamed datagram processing4007. These processes are substantially identical to processes 1005,1006 and 1007 respectively.

Process 4008 performs background processing. This is similar to process1008, except that process 4008 has no step corresponding to step 2707,at which the real time data server 502 informs the application server501 of the network conditions. The only substantial difference betweenthe client and the server is that the client does not perform a processcorresponding to session maintenance 1009.

An additional difference is that, in general, a session will berequested and terminated by the user of PDA 511.

Datagram reception process 4005 includes step 4009, at which a resendlatency value is calculated, and background processing 4008 includesstep 4010, at which a heartbeat rate is negotiated. These stepscorrespond to steps 1909 and 2702 respectively. Although the facilityfor these steps exists on both the real time data server 502 and PDA511, in practice, in this embodiment, it is only PDA 511 that uses them.They are thus described in FIG. 41 and FIG. 42 respectively.

FIG. 41

FIG. 41 illustrates resend latency measurement. This is the delay causedby having to resend a datagram, as opposed to the connection latencywhich is the delay caused by the network. Packets sent across theInternet 110 are not guaranteed to arrive, which is why an additionalprotocol like MTP must be used to ensure eventual delivery. When an MTPdatagram gets “lost”, meaning that it is not acknowledged, it will beresent. The data it contains, therefore, is more out-of-date than itwould have been had it arrived first time. This resend latency iscalculated at step 4009.

In FIG. 41 the original datagram 4101 is transmitted and fails to bedelivered. After a time, either through a lack of acknowledgement or anegative acknowledge, the real time data server 502 will resend thedatagram. The resent datagram 4102 is also lost. A third attempt 4103 issuccessful.

Each datagram contains an elapsed time field 4104. In datagram 4101 thisis set to zero. In datagram 4102 it is the difference between thetransmission time of datagram 4102 and the transmission time of datagram4101; similarly for datagram 4103. Thus, for example, the elapsed timefield for datagram 4103 is 421 milliseconds.

When a resent datagram is received the resend latency is recalculatedusing a smoothing filter on the elapsed time. If no datagrams arereceived at all then the resend latency is gradually increased. Thisoccurs in this embodiment once a heartbeat period has passed with noreceipt of datagrams. However, receipt of any datagram, includingtransactional datagrams and empty streamed datagrams, will at this pointdecrease the latency, since it implies that the reason for non-receiptof streamed data may be that there is no data to send, and thus the lastreceived updates may still be current.

The resend latency is added to the connection latency to give theapplication latency. This is the actual time delay of the data displayedto the user on PDA 511. Thus the timeliness of the data, according to afunction of the length of time taken to reach the client and thepossible number of resends it required, is displayed to the user toallow him to make a decision regarding whether or not to use the data.Optionally, when the application latency falls below a certain thresholdthe screen may “grey out” and transactions may be suspended.

FIG. 42

FIG. 42 details step 4010, at which the PDA 511 negotiates a newheartbeat rate with real time data server 502. The heartbeat rate is themaximum interval that is allowed to pass without sending data, both bythe server and by the client. If no data has been sent at the end ofthis interval then an empty streamed datagram is sent. In thisembodiment, this is combined with the connection latency measurement bysending the latency measurement datagram at intervals which are the sameas the heartbeat rate. If the server does not receive any data from theclient for an interval substantially equal to twice the heartbeatinterval, then the session will stall. The client, however, does notstall a session on non-receipt of data, but continues to send data, orheartbeats if there is no data. A heartbeat is in this embodimentusually a latency-measurement datagram, but could be an empty datagramwith the KAL field 1213 set.

Since latency measurements are sent at the heartbeat rate, the latencyis more accurate when the heartbeat is faster. This means that when theuser is, for example, trading, the heartbeat should be fast, whereaswhen he is browsing news stories the heartbeat should be slow. Thus theheartbeat negotiation is triggered by events that occur when the PDA 511switches applications, minimises or maximised applications or enters aparticular state in an application.

At step 4201 a new heartbeat rate is requested by sending a datagramthat has SYN field 1202 set and a number in acknowledgement number field1212, but does not have ACK field 1203 set. At step 4202 a question isasked as to whether the heartbeat rate has been agreed by receiving anacknowledgement of this datagram. If this question is answered in thenegative then the heartbeat rate is not changed. Alternatively, if theheartbeat rate is agreed, the rate is changed at step 4203.

Associated with this is the possibility that the client may at any timechange its application requirements. For example, on minimising of thedisplay of stock prices the client may, using a transactional datagram,change its application requirements to stop the transmission of stockprices. On using the telephone, which requires as much bandwidth aspossible, the client may change its application requirements to ceaseall transmission of streamed data. When the user returns to the displayof stocks then the application requirements can be changed again toindicate that the default requirements apply. However, even when nostreamed data is being sent, the client and server continue to sendlatency measurements at the agreed heartbeat rate. This indicates notonly that the connection is still active but allows an immediate displayof latency when the user returns to the display of streamed data.

1. A method of communicating between a server and a terminal, whereinsaid terminal establishes a first communication path with said servervia a first allocated address; the server allocates an identificationreference to a communication session upon initiation of saidcommunication session, wherein during said communication session saidfirst communication path is closed and replaced by a secondcommunication path with said server via a second allocated address; andthe server maintains said communication session after replacement of thefirst communication path with the second communication path byidentifying said identification reference and communicating with saidremote station via said second allocated address.
 2. A method accordingto claim 1, wherein said communication path includes a first radionetwork, and said first communication path is closed and replaced bysaid second communication path when said terminal stops using said firstradio network and starts using a second radio network.
 3. A methodaccording to claim 1, wherein said communication path includes a mobiletelephony network, and said first communication path is closed andreplaced by said second communication path when said second terminalmoves between cells of said mobile telephony network.
 4. A method ofcommunicating between a server and a terminal, wherein said terminalestablishes a first communication path with said server via a firstallocated address; the server allocates an identification reference to acommunication session upon initiation of said communication session; andduring said communication session said first communication path isclosed and replaced by a second communication path with said server viaa second allocated address; wherein the server maintains saidcommunication session after replacement of the first communication pathwith the second communication path by identifying said identificationreference and communicating with said remote station via said secondallocated address; and said communication path includes a first radionetwork, and said first communication path is closed and replaced bysaid second communication path when said terminal stops using said firstradio network and starts using a second radio network.
 5. A serverconfigured to transmit data packets to at least one terminal, whereinsaid terminal establishes a first communication path with said servervia a first allocated address; the server allocates an identificationreference to a communication session upon initiation of saidcommunication session; during said communication session said firstcommunication path is closed and replaced by a second communication pathwith said server via a second allocated address; and the servermaintains said communication session after replacement of the firstcommunication path with the second communication path by identifyingsaid identification reference and communicating with said remote stationvia said second allocated address.
 6. A server according to claim 5,wherein said communication path includes a first radio network, and saidfirst communication path is closed and replaced by said secondcommunication path when said terminal starts using a second radionetwork.
 7. A server according to claim 5, wherein said communicationpath includes a mobile telephony network, and said first communicationpath is closed and replaced by said second communication path when saidterminal moves between cells of said mobile telephony network.
 8. Aserver according to claim 5, configured to transmit data packets to andreceive data packets from a plurality of terminals.
 9. Instructionsexecutable by a network of computers and/or programmable data processingdevices such that when executing said instructions stations connected tosaid network will transmit data packets over said network and saidnetwork will perform the steps of establishing a first communicationpath between a first station and a second station via a first allocatedaddress; allocating an identification reference to a communicationsession upon initiation of said communication session, wherein duringsaid communication session said first communication path is closed andreplaced by a second communication path via a second allocated address;and maintaining said communication session after replacement of thefirst communication path with the identification reference andcommunicating with said remote station via said second allocatedaddress.
 10. Instructions according to claim 9, wherein saidcommunication path includes a first radio network, and said firstcommunication path is closed and replaced by said second communicationpath when said terminal stops using said first radio network and startsusing a second radio network.
 11. Instructions according to claim 9,wherein said communication path includes a mobile telephony network, andsaid first communication path is closed and replaced by said secondcommunication path when said terminal moves between cells of said mobiletelephony network.
 12. Instructions executable by a combination of aserver and mobile terminal connected to a said server by a radio networksuch that when executing said instructions said combination performs thesteps of at a terminal, establishing a first communication path with theserver via a first allocated address; at the server, allocating anidentification reference to a communication session upon initiation ofsaid communication session; wherein during said communication sessionsaid first communication path is closed and replaced by a secondcommunication path with said server via a second allocated address; andthe server maintains said communication session after replacement of thefirst communication path with the second communication path byidentifying said identification reference and communicating saididentification reference and communicating with said remote station viasaid second allocated address.
 13. Instructions according to claim 12,wherein said communication path includes a first radio network, and saidfirst communication path is closed and replaced by said secondcommunication path when said terminal stops using said first radionetwork and starts using a second radio network.
 14. Instructionsaccording to claim 12, wherein said communication path includes a mobiletelephony network, and said first communication path is closed andreplaced by said second communication path when said terminal movesbetween cells of said mobile telephony network.
 15. A computer readablemedium having computer readable instructions executable by a computersuch that when executing said instructions a computer will perform thesteps of: responding to a first communication path established by anetworked device via a first allocated address; allocating anidentification reference to a communication session upon initiation ofsaid communication session, wherein during said communication sessionsaid first communication path is closed and replaced by a secondcommunication path via a second allocated address; and maintaining saidcommunication session after replacement of the first communication pathwith the second communication path by identifying said identificationreference and communicating with said remote station via said secondallocated address.
 16. A computer readable medium having computerreadable instructions according to claim 15, such that when executingsaid instructions a computer will perform the step of using a secondradio network wherein said first communication path includes a firstradio network, and said first communication path is closed and replacedby said second communication path when said terminal stops using saidfirst radio network and starts using a second radio network.
 17. Acomputer readable medium having computer readable instructions accordingto claim 15, such that when executing said instructions a computer willperform the step of responding to movement of a terminal, wherein saidcommunication path includes a mobile telephony network, and said firstcommunication path is closed and replaced by said second communicationpath when said terminal moves between cells of said mobile telephonynetwork.